Technology relating to handheld flashlights incorporating a direct-current power supply in the form of replaceable batteries and low-voltage, incandescent bulbs achieved a technological plateau in the 1970s. Advances in the state of the art typically related to methods of packaging the batteries and bulbs, and reflector designs. In particular, the capabilities of flashlights of this type are strictly limited by inherent characteristics of the incandescent bulb itself. Initially, evacuated bulbs using tungsten filaments enabled power supplies in the range of 1.3V (and more when such batteries are connected in series) to provide varying levels of illumination. So-called halogen bulbs permitted higher filament temperatures, increasing the output of such flashlights. Nevertheless, the inherent inefficiency of incandescent bulbs limited the duration of operation of such flashlights to a matter of a few hours or less, depending on the number of dry cells provided in the power supply. That is, for increased run time the batteries could be connected in parallel. For increased light intensity the batteries could be connected in series (for increased voltage) but at the expense of run time. In addition, filament bulbs are highly susceptible to mechanical shock, breaking the filament and rendering the flashlight inoperative. In addition, substantial development effort was directed to switch mechanisms for intermittently connecting the direct current power supply to the incandescent bulb so as to render either a more reliable or inexpensive switch, or both.
U.S. Pat. No. 4,242,724 to Stone is believed to be representative of one evolutionary branch of such technology relating to the packaging of a disposable floating flashlight in which the outer casing of the light itself forms a part of the switch mechanism that, when squeezed, completes electrical continuity between two AA (1.3 V each) batteries and an incandescent bulb. The flashlight is compact, and floats if accidentally dropped into water. U.S. Pat. No. 5,134,558 to Williams et al. discloses a different evolutionary branch in which the voltage output from four AA-type batteries is boosted by an oscillator-driven transformer rectifying circuit to an intermittent high voltage applied to a xenon gas flashtube so as to provide a high-intensity emergency flasher. The device disclosed in Williams et al. delivers significantly more illumination from a direct current power supply than does the incandescent bulb type of flashlight disclosed by Stone. Nevertheless, the circuitry disclosed in Williams et al. for operating the xenon flashtube is expensive, bulky, and only suitable for intermittent operation of the flashtube rather than for providing a constant light output. Thus, the teaching of the prior art disclosed by Williams et al. is not suitable for remedying the inherent limitations of the incandescent bulb type of flashlight technology disclosed by Stone.
As stated above, the fundamental limitations of prior art flashlights are related to inherent limitations of incandescent bulb technology, and inherent limitations of electrical circuits for driving other light-generating devices, such as the xenon flashtube shown by Williams et al. Nevertheless, semiconductor technology contemporarily advanced so as to provide semiconductor devices, including light-emitting diodes (hereinafter occasionally “LEDs”) having significantly lower current drain than incandescent bulbs in a highly robust package operable at relatively low direct current voltages. In addition, early LEDs were substantially more power efficient than incandescent bulbs having similar current consumption characteristics. Finally, the small physical size of LEDs permitted extremely efficient packaging shapes to be adopted for such lights. U.S. Pat. No. 5,386,351 to Tabor discloses such a space-efficient packaging design for a single LED flashlight. The Tabor patent discloses a two-part, snap-fit housing incorporating a discoid type of battery in which one leg of a two-terminal LED is employed as part of a cantilever spring switch mechanism that, upon depression by a flexible button, completes a direct current circuit to the LED. Unfortunately, such early stage LEDs could not provide significant light output without being driven at very high currents, in which case, the power efficiency of the LED with respect to the quantity of light produced significantly decreased. Also, LEDs in use during the period in which the Tabor patent application was filed were capable of producing light in only the red part of the visible spectrum. These two limitations resulted in an LED flashlight only having utility for intermittent operation or continuous illumination over short distances. Therefore, such personal flashlights could not supplant conventional incandescent bulb flashlights, which have a more linear relationship with respect to supply voltage and current. A high-intensity incandescent bulb flashlight can be produced by merely increasing the amount of current and/or voltage supply to the bulb. Conventional LEDs, being nonlinear devices, do not respond in such a linear fashion. Therefore, LEDs were often employed in lighting devices for alternative purposes, such as the color-coded, multiple-LED light and key device shown in U.S. Pat. No. 4,831,504 to Nishizawa et al. The Nishizawa et al. patent discloses a combination LED flashlight and key in which multiple LEDs having different colors are driven by separate, manual switches and/or a microprocessor to signal an appropriate light-detecting and demodulating device in association with a door lock or operating lock. Similarly, international Patent Application No. WO 01/77575 A1 titled, “Portable Illumination Device” published on Oct. 18, 2001, to Allen discloses a unique product package for a single-LED personal flashlight employing a discoid type of battery in which multiple depressions of a switch incorporated into the product housing cycle the single LED through multiple modes according to instructions stored in a microprocessor within the housing. Neither the invention disclosed by Nishizawa et al. nor the invention disclosed by Allen is capable of substantially increasing the light output of the LED such that the lighting devices disclosed therein are adequate replacements for high-intensity incandescent bulb flashlights. The principal reason for this is that light-emitting diodes, being junction semiconductor devices, have a forward bias voltage that is predetermined by the physics of the semiconductor materials from which diodes are manufactured. The forward-biased voltage of silicon-based light-emitting diodes is approximately 3.6 V for aqua, blue, and white LEDs and 1.8 V for red, yellow, and green LEDs. The voltage-current characteristics of devices of this type are such that substantially increasing the applied voltage outside of a range defined by the forward bias voltage does not substantially increase the light output of the device, but merely results in vastly increased current flowing therethrough. The power output of a diode being equivalent to the product of the voltage applied thereto and the current flowing therethrough, higher voltages on the power supply side merely result in much higher current which results in wasted power without significant additional illumination. Thus, the light-emitting diode can basically be characterized as a device having an optimal operating characteristic defined by a substantially constant current at a nearly fixed voltage. Therefore, the only efficient method for substantially increasing light output of a prior art LED device based on the silicon architecture is to provide multiple LEDs in parallel with the direct current voltage supply. Unfortunately, this arrangement only drains the typical (1.2, 1.5, or 3 V) battery supplies quickly until the batteries can no longer supply the forward bias voltage of the diodes. Placing the LEDs in series with the power supply merely exacerbates this problem. Thus, although the direct current power supply may be capable of providing additional current (i.e., the batteries are not fully discharged yet) the partially depleted batteries cannot forward bias and thus illuminate the LEDs.
The semiconductor industry has recently addressed the above limitations of LEDs by providing white light LEDs based on indium-gallium-arsenic-phosphide architecture having forward bias voltages in excess of 3.6 V. LEDs of this type not only provide a white light that is more effective than the red light of the prior art doped-silicon technology, but also produce substantially more light output for a given current. Unfortunately, the battery technology based on a voltage of approximately 1.5 V per dry cell is again limited in that three dry cells in series, having a nominal voltage of 4.5 V, are quickly drained to an actual applied voltage of less than 3.6 V at which point the white light LED becomes inoperative even though the batteries still retain a substantial charge.
U.S. Pat. No. 7,015,654 to Kuhlmann et al., assigned to the assignee of the present invention, addresses the need for an LED driver circuit that conditions all of the available power within the conventional dry cell battery for application to high forward bias voltage LEDs by providing a microcontroller and boost converter circuit providing constant current to a light-emitting diode or diode array. The microcontroller is operatively coupled with a semiconductor switch and the boost converter circuit so as to measure the ability of a DC power supply to charge an inductor of the boost converter circuit. Duty cycles of the semiconductor switch are modified according to measurements so as to supply substantially constant current to the LED or LED array through the inductor, regardless of the actual instantaneous battery voltage. The problem of unused battery charge in LED flashlights having been solved, a challenge remained in making such flashlights rechargeable with the battery(s) in situ.
Rechargeable miniature flashlights are known and one flashlight of this type is described in U.S. Pat. No. 6,457,840, to Maglica et al., issued on Oct. 1, 2002. This rechargeable flashlight is of the incandescent bulb type utilizing conventional, miniature two- or three-cell flashlight batteries. The rechargeable flashlight has an external, tailcap switch that enables an external, conventional charging device to establish a current path to the internal rechargeable batteries. The external charging device uses a conventional voltage regulator to “step down” the power source voltage to the internal battery voltage. The only internal circuitry provided within the flashlight itself is a single diode to reverse block current flow from the internal batteries to external “charge rings 63, 70” (so that they do not) become inadvertently shorted together such as by laying the flashlight down on a metallic surface, in contact with a coin, etc. However, this prior art flashlight is not of the LED type and does not teach how to effectively recharge such a flashlight with internal circuitry and batteries in situ.
Thus, a need exists for a rechargeable light-emitting diode flashlight having self-contained recharging circuitry.
A further need exists for a self-contained, rechargeable light-emitting diode flashlight capable of recharging itself from a low-current power supply.